The vibrating member for use in an acoustic transducer, such as a diaphragm of a loudspeaker or a microphone, or a cantilever of a pickup cartridge of a disc-record player, is usually constructed of aluminum or titanium etc. which is light in weight and easily formable.
For the diaphragm of aluminum or titanium, there is a vibration mode produced which is spread throughout the diaphragm and provides a unique peak in its high frequency characteristics causing the reproduced sound quality to be considerably degraded.
When the vibrating member is a cantilever, it occupies a major part of the effective mass of the vibration system, so that it becomes difficult to minimize the effective mass of the vibration system to improve the performance of the cartridge except by reducing the mass of the cantilever. However, if the wall thickness and the diameter of the cantilever are reduced to minimize the weight thereof, the rigidity thereof is necessarily reduced causing the overall characteristics of the cantilever to be degraded.
In order to overcome the above mentioned disadvantages of the conventional vibrating member, an inorganic material having a large ratio of Young's modulus E to density .rho. should be used in fabricating the vibrating member. Examples of such material include boron, beryllium etc. Boron, particularly of high purity, is relatively difficult to obtain and beryllium can not be prepared without incurring large expenses to install equipment for preventing public pollution. Moreover, boron and beryllium are difficult to form mechanically. It has been proposed to form a vibrating member consisting solely of boron or beryllium, and thereby avoid the latter difficulty, by forming a coating layer of beryllium or boron on a suitably configured substrate of easily formable material and thereafter separating the coating layer from the substrate. The coating layer thus obtained is ready to use as a diaphragm or the like according to the specific shape of the substrate. However, the coating layer which would be provided by a vapor-deposition process is mechanically fragile. Moreover, an expensive installation, including electron beam apparatus and other equipment, is necessary to provide such coating on the substrate. In addition to this, the time required to provide the coating is considerably long. Therefore, the use of boron or beryllium, alone, for mechanically forming the vibrating member has not been practiced.
Therefore, it had been proposed to use aluminium or titanium, which is easily formable, as a substrate of the vibrating member and to coat the substrate with boron or beryllium by means of physical evaporation or chemical processing to form a coating layer. As is well known, in order to obtain good properties of the deposited layer and a good adhesion thereof to the substrate, it is desirable to heat the substrate to a temperature higher than 150.degree. C. However, since the thermal expansion coefficiencies of the coating layer and the substrate are usually very different, there is a strong possibility of mechanical deformation of the substrate and/or cracking of the coating layer by a subsequent cooling to the normal temperature after the heating and deposition process.